The problem is that the movie makers are getting their terms mixed up, the Sea People in LWW were portrayed as mermaids, even though they are supposed to look just like people. The Water Nymphs like the dryads were just the spirits of the rivers while the Sea People were just ordinary people that lived under water. Also it could not be a nymph because the sons of Frank and Helen married the Water Nymphs which means they could come out of the water, something a mermaid could not do.

I won't get involved in the whole Naiad/Nymph/SeaPeople debate right now, so i'll just call it a "Water Thing" here: Anyway, given where the Movie Story Book says the "Lucy Waving at The Water Thing" scene occurs, i am now fairly certain it is just an incidental moment of no real significance which occurs during the main credit montage at the beginning of the film.

Like the previous films, i imagine the credits sequence will occur a little way into the movie, and the most obvious and dramatic place to put the main title would be shortly after we arrive on the Dawn Treader. Such a placement also provides potential for a nice 3 minute window with no dialogue, and just showing various bits and pieces of life on the boat whilst the main title music plays - Again such as with PC and LWW, which both had "Journey Sequences" during the credits.

I agree about calling the water nymph/mermaid/naiad/sea peson a "water thing". haha!

But I always pictured the credits playing out right away, as Lucy and Edmund ride their bikes with the groceries back to the Scrubbs' house.